ultramarine or blue

I've been puzzling over this (Scott A3) and the following Scott A5 for some time.
I've noticed that those with whiter paper nearly always have a bluer shade. I suspect that in soaking, some of the ultramarine stamps go toward blue,(the red being more fugitive) confusing the situation, so that with a large collection of used stamps you would get a continuum of shade differences.

Thanks all who participated in the poll and/or helped out!  Special thanks to whoever voted for "orange".  Great minds think alike!

OK, mystery solved (I think). I decided if you can't climb over the mountain, then go around it. So I went through the NVPH catalog really carefully, to see if there might be another way to get a final specialized ID on the stamp without requiring color identification.

Turns out, in this case, there is!!!

There are 2 types of numbers for the stamps in this series (Type I & Type II). The stamp shown is Type II.T he perf 13½ variety with Type II only comes in the blue color. Therefore, the color shade must be blue.

For perf 13½, only Type I comes in ultramarine, so that also indicates that the color shade cannot be ultramarine.

In the NVPH, the stamp would be #7 IIC, which is the cheapest of all the varieties (it figures )

The NVPH is in Dutch, a language in which I have absolutely no background. So maybe a Dutch collector or a Dutch speaker (maybe SWH ?) can chime in and correct/verify that it must be the blue color shade?

I guess there are always exceptions... smiley-face-whistle-2